The production of concrete masonry units is accomplished using a concrete mold assembly and a tamperhead which strips formed and compacted concrete or other medium from a mold cavity. The tamperhead is composed of several sub-components which include an upper head structure, a plunger and a stripper shoe. Multiple sets of stripper shoes and plungers may be connected to a single head structure and used to strip multiple masonry units from the mold assembly or set of concrete mold cavities. The plungers are commonly fabricated in structural shapes from a rigid material such as steel and provide the structural load path to compress the concrete and strip the formed concrete product from the mold.
The production or forming process induces significant wear and stress on the plunger. Upon filling the mold with concrete, the tamperhead is lowered until the stripper shoe contacts the concrete. The stripper shoes are guided and forced into alignment with the mold cavities by leading angles or chamfers on the top edge mold cavities. As the stripper shoes are lowered, the impact of the stripper shoes with the leading angles imparts high stresses on the plunger, especially the joint attaching the plunger to the head structure.
The forming process also includes vibrating or shaking the mold assembly with a vibration system in order to further compact the concrete. The vibration system spreads the concrete material evenly within the mold assembly cavities to produce a more homogeneous concrete product and assist in compacting the concrete product. Vibrations from the mold assembly transfer to the stripper shoes and consequently to the plunger and head structure and occur approximately every ten to fifteen seconds during typical production
Unfortunately, the repeated forces transmitted by the vibrations from the mold to the stripper shoe makes the plunger and joints susceptible to fatigue failure. Furthermore, the high impact stresses from the alignment of the stripper shoe with the mold cavity further stress the plunger and joints. As a result of the combined stresses, expensive plungers typically last only short periods and must be replaced at great expense and a loss of production time.
Furthermore, as the vibrator system shakes the mold assembly, the rest of the product-forming machine also experiences vibrations as forces are transmitted through the plunger. This vibration fatigues the machine parts and alters the clearances between moving parts, such as hydraulics and gears. Mold assemblies and stripper shoes also suffer from repeated impact stresses and wear during vibration and alignment. As molding components degrade, surface quality and product density of the finished product degrades. Thus, transmitted vibrations and alignment impacts reduce machine and mold assembly operating life, resulting in reduced product quality and increased replacement of parts.
The prior art teaches a traditional approach of avoiding frequent failures and replacements of plungers by consistently shortening the plunger length and increasing the plunger strength and/or stiffness. However, this approach has not been successful at extending the useful life of a plunger. Time has shown that short stiff plungers still frequently fail, with the joint between the plunger and the head structure being especially vulnerable. In fact, stiffer plungers increase wear on stripper shoes and mold assemblies and therefore exacerbate the need to replace or repair expensive components.
Traditional plungers with reduced flexibility also increase production costs. As the flexibility of traditional plungers decreases, the weight and/or expense of fabricating plungers increases as a result of increased thickness or design. Increased weight functions to increase the required power and expense of running the production machinery and to decrease the resonant frequency of the plunger and stripper shoe. The increased weight also intensifies the deterioration of moving parts under heavy load and increases impact forces between stripper shoes and molding assemblies. Although lighter plungers may be constructed from materials with high strength to weight ratios, the additional cost of materials and fabrication has been prohibitive.
Therefore, there exists a need for a tamperhead and mold assembly which is less susceptible to failure from vibration, reduces fatigue stresses in the connection between the head structure and plunger, and reduces impact loads between mold cavities and stripper shoes during alignment of stripper shoes and mold cavities and during vibration.
There is also a need to improve surface quality and product density of the finished product by extending the useable life of the molding components and machinery.